Grossenbacher—Radial Growth in Trees . 
56 
The “normal” bark pressure was determined by stretching 
rings of bark over a smooth cylinder by means of weights until 
the bark had attained the length it had while still attached to 
the tree. In his later work 113 the rings of bark were straightened 
out and weighted at one end to determine the force required to 
stretch the bark to its former length, for it was found that the 
results obtained in this way were the same as those gotten with 
the more elaborate apparatus. The bark pressure of conifers was 
found to be usually under one-half an atmosphere and that 
of broad-leaved trees about twice as great. In case of conifers 
the pressure seemed to increase in fall on an average about 
0.8 gm. per square millimeter of cross section, while the average 
of similar measurements on a number of broad-leaved trees indi¬ 
cated a decrease of pressure in fall equal to 12.5 gm. per square 
millimeter of cross section. He maintained that the breaking 
strain of bark is never reached by growth pressure. Bark pres¬ 
sure was found greatest in regions of most rapid radial growth, 
for instance on the side of excentric stems with the longer radius. 
By using pressures from five to eight atmospheres the sum¬ 
mer-wood type of radial growth was induced in spring on trees 
having comparatively little difference in the size of spring and 
summer-wood cells, while on trees having very marked differ¬ 
ences between spring and summer wood it was practically impos¬ 
sible to induce the formation of the summer-size of cells in 
spring by increasing the bark pressure. In reducing the bark 
pressure by means of longitudinal slits in the outer bark in sum¬ 
mer, typical spring wood vessels developed in trees which nor¬ 
mally have only a slight difference between size of spring and 
summer wood cells; but in trees like Quercus and Fraxinus in 
which a marked difference occurs between spring and summer 
wood, the spring wood vessels could not be thus induced. 
Krabbe therefore concluded that bark pressure remains practi¬ 
cally the same throughout the growing season and that changes 
in bark pressure could not be the cause of ring formation be¬ 
cause it requires such a great increase to influence the size of the 
wood cells. 
118 fiber das Wachsthum des Verdickungsringes und der jungen Holz- 
zellen in seiner Abhangigkeit von Druekwirkungen. Abhandl. Kgl. 
Akad. Wisa. Berlin. 1884. Anhang. 1:1-80. 1885. 
